The Complicated Origin of Earth's Water Jun Wu and colleagues at Arizona State University have examined the issue broadly, considering planetary accretion, magma oceans, core formation, mantle convection, and interaction of a proto-atmosphere with the surrounding solar nebula. Their comprehensive hypothesis uses the isotopic composition of hydrogen as a tracer of the sources of water and its processing during planetary formation and subsequent differentiation. Here is an overview of their model in six stages. Overview: Water delivery and redistribution to Earth was a complicated business with hydrogen coming from chondritic and solar nebular sources. Summary: A major mystery in planetary science is the origin of Earth's water. Using the deuterium/hydrogen ratio (D/H) as an isotopic tracer of the origin of water in our planet, Jun Wu (Arizona State University) and colleagues at ASU model the origin of water to Earth. Through quantitative modeling they show that Earth contains about eight-oceans-worth of water, of which about five-oceans-worth reside in the metallic core (as hydrogen). They also show that a minor but important component came via "ingassing" from a proto-atmosphere surrounding Earth, which formed by accretion of solar nebula gas. It would not take much dissolution of hydrogen from this atmosphere to modify the hydrogen isotopic composition of the originally accreted Earth to account for the overall isotopic composition (given by D/H) of the planet. The model also considers dissolution of hydrogen into the metallic core of Earth and its effect on D/H. The research is a good example of the power of using isotopic compositions as tracers of planetary formation and differentiation processes. Reference: Wu, J., Desch, S. J., Schaefer, L., Elkins-Tanton, L. T., Pahlevan, K., & Buseck, P. R. (2018) Origin of Earth's Water: Chondritic Inheritance Plus Nebular Ingassing and Storage of Hydrogen in the Core, Journal of Geophysical Research: Planets, v. 123, p. 2691-2712, doi: 10.1029/2018JE005698.

1−2 Million Years After CAI formation: Planetary embryos in early Solar System 0.1 ME 0.1 ME D/H = 140 x 10-6 (chondritic ratio) 0.4 ME STAGE 1: The first step occurred long before there was an Earth, about 1–2 million years after formation of CAIs, the first solids to form in the Solar System. During this time large objects (called planetary embryos) assemble with masses up to a few tenths of Earth's final mass. The idea of such rapid growth is relatively new and is driven by two ideas. One is called streaming instability, which leads to growth of meter-sized aggregates to 100-km planetesimals in less than a million years. The other is nicknamed pebble growth, which starts with meter-sized (or smaller) objects and ends up with large planetary embryos in less than a million years. This short time scale is supported by age dates on assorted types of differentiated meteorites and by the time of core formation in Mars (based on tungsten isotopes in Martian meteorites), only 1.8 million years after CAIs (as shown by Nicholas Dauphas and A. Pourmand in 2011). So, Wu and coworkers assume that it takes only 1-2 million years to form a bunch of planetary embryos. They also assume that the D/H in them is the same as observed in water-bearing minerals in carbonaceous chondrites, about 140 x 10-6 (it could range from 130–170 x 10-6). ME = Mass of Final Earth Planetary embryos contain enough water to make 7 to 8 oceans in the final assembled Earth. http://www.psrd.hawaii.edu/Dec18/origin-earth-water.html (Figure based on Wu et al., 2018, JGR:Planets, doi: 10.1029/2018JE005698.) 1

~2 Million Years After CAI formation: Cores form in planetary embryos Mantles: D/H = 150−160 x 10-6 Cores: D/H = 125−135 x 10-6 Mantles are molten (magma oceans) STAGE 2: Then the planetesimals melted. Growing planetesimals began heating as they were forming. The heat came from the decay of 26Al, one of the isotopes of aluminum, an abundant element. When radioactive elements decay, they release a particle such as an electron and some energy, which heats up its surroundings. 26Al has a half-life of only 730,000 years, which means, of course, that this original 26Al is gone now. (We know it existed from excesses in the amount of 26Mg in aluminum-bearing minerals in chondrites and other ancient materials.) The short half-life makes 26Al a potent heat source, so strong that it can melt a planetesimal as small as about 10 kilometers in radius. Larger planetesimals melt readily, allowing metallic iron in them to drain to their cores. As the metallic dribbling begins, some hydrogen readily dissolves in metal droplets. Wu and coworkers calculate, based on published hydrogen solubility in metallic iron, that the cores of the planetary embryos would contain about 0.025 to 0.055 wt% hydrogen, which corresponds to three- to six-oceans-worth in the metallic core of the final assembled Earth. In addition, H and D do not enter the metallic core in equal proportions. Based on experiments on H and D dissolution and the chemical principles of isotopic fractionation, Wu and colleagues conclude that the cores of the embryos would have D/H of 125–135 x 10-6. This lower ratio would increase the D/H in the mantles of the embryos to 150–160 x 10-6. Heating by 26Al causes melting and sinking of metallic iron to form cores in planetary embryos. About 60% of the hydrogen dissolves in cores. Metallic iron takes in hydrogen with lower D/H, resulting in higher D/H in the overlying mantle. http://www.psrd.hawaii.edu/Dec18/origin-earth-water.html (Figure based on Wu et al., 2018, JGR:Planets, doi: 10.1029/2018JE005698.) 2

2−3 Million Years After CAI formation: Largest embryo forms and retains an atmosphere proto-atmosphere: D/H = 21 x 10-6 H2 magma ocean: D/H = 109 x 10-6 mantle: D/H = 150−160 x 10-6 The proto-atmosphere contains hydrogen from the solar nebula, which has low D/H. It diffuses into the magma ocean, lowering its D/H and providing about 0.14 oceans of H2. This process is called “ingassing.” STAGE 3: A planetary embryo surrounded by an ocean of magma could attract an atmosphere from the solar nebula, forming a stable (though evolving) proto-atmosphere. Jun Wu and colleagues suggest that this atmosphere could interact with the magma ocean to incorporate water with a lower D/H than that produced during stages 1 and 2. This process has been dubbed "ingassing." Most investigators have concluded that this mechanism is not particularly effective. Exceptions are papers by Marc Hirschmann (University of Minnesota) and colleagues in 2012 and Zack Sharp (University of New Mexico) in 2017. However, Jun Wu and his coworkers show that this process could be effective enough to lower the average D/H in the magma ocean surrounding a planetary embryo to about 109 x 10-6. It would provide about 0.14 oceans of hydrogen depending on the magnitude of partitioning of H and D between iron melt and silicate mantle. The exact amount of water in the magma ocean is uncertain, hence the D/H ratio is also uncertain. Nevertheless, it is likely that D/H in the magma ocean would have been lower than originally present in a planetary embryo. The ingassing process would have operated early, within 3 million years after CAI formation. This is the time the solar nebula lasted, as shown by astronomical observations of disks around young stars and by the time of Mars formation (1.8 million years after CAIs). http://www.psrd.hawaii.edu/Dec18/origin-earth-water.html (Figure based on Wu et al., 2018, JGR:Planets, doi: 10.1029/2018JE005698.) 3

>3 Million Years After CAI formation: Magma ocean overturn delivers material with low D/H to core-mantle boundary magma ocean: D/H = ~150 x 10-6 Cumulates that interacted with atmosphere: D/H = ~109 x 10-6 (thickness exaggerated) 0.4 ME STAGE 4: A magma ocean does not simply solidify into a rock-solid mantle. The crystallization process produces an unstable pile of rock, with some admixed melt, with denser minerals on top and less dense ones below. The consequences of this density stratification is that the pile overturns. It turns over in a decided messy way. The overturn would mix patches of magma ocean with differing D/H due to incomplete mixing of the low D/H hydrogen in the proto-atmosphere into the magma ocean and the complicated dynamics of the overturn. Some crystallized piles of crystals (called cumulates) would have acquired the magma ocean's D/H (estimated by Wu and colleagues to have D/H of 109 x 10-6) and could have settled to the core-mantle boundary. These might later rise in plumes to form lavas with high 3He/4He and low D/H. Highly convecting magma ocean (gray) overturns and mixes materials with low D/H throughout the mantle. However, some low D/H melt or rock sinks to the core-mantle boundary. Earth continues to grow by accretion. http://www.psrd.hawaii.edu/Dec18/origin-earth-water.html (Figure based on Wu et al., 2018, JGR:Planets, doi: 10.1029/2018JE005698.) 4

10−100 Million Years After CAI formation: Earth continues to grow through accretion of planetary embryos Growing Earth Earth ends up with ~3 oceans of water in the mantle and an average D/H of 150 x 10-6 STAGE 5: During the next 10 to 100 million years Earth continued to grow through accretion. The accretion process is not gentle as the objects smash into each other, mixing their mantles and cores, possibly smearing out the low D/H cumulates along the core-mantle boundaries. The cores of the impacting embryos merge, their mantles mix, their cumulate layers with low D/H get smeared out along the core-mantle boundary. http://www.psrd.hawaii.edu/Dec18/origin-earth-water.html (Figure based on Wu et al., 2018, JGR:Planets, doi: 10.1029/2018JE005698.) 5

Mantle plumes drag up material with low D/H from core-mantle boundary Today: Mantle plumes drag up material with low D/H from core-mantle boundary Oceans D/H = 156 x 10-6 1.0 ocean Mantle D/H = 150 x 10-6 Mantle Plume 2.0 oceans Ultra Low Velocity Zone? D/H = 109 x 10-6 STAGE 6 (TODAY): From about 4.4 billion years ago to now, Earth has done its Earth thing, forming continents, starting plate tectonics, moving water around inside and on top of it. Some surviving regions with low D/H deep in the mantle were incorporated into ascending plumes of buoyant rock that rose up to eventually melt and produce basalts like we see in Baffin Island, Canada. Most magmas that erupt onto the surface of our planet reflect more average mantle, having D/H around 150 x 10-6. Wu and coauthors estimate that Earth contains almost eight-oceans-worth of water in it, about 60% of it in the metallic core (dissolved as hydrogen). Core: D/H = ~130 x 10-6 4.8 oceans http://www.psrd.hawaii.edu/Dec18/origin-earth-water.html (Figure based on Wu et al., 2018, JGR:Planets, doi: 10.1029/2018JE005698.) 6